As I look at the unit screen, you've got an A5M4 Claude unit upgrading to the same a/c hence why you don't see the ability to upgrade but also what aphrochine said on PDU on/off. If off then you're stuck if it has no upgrades beyond the Claude.

Well I mean is it more of a disadvantage for Japan or for the Allies? With PDU off I'd imagine not every single bomber squadron will be converted to a B-29, and not every single fighter squadron will be P47/P59H/Corsair/P80

I'm playing with PDU ON. I've upgrading an A5M4 group to A6M2 earlier in the game. Maybe because it's going to withdraw. So probably don't want to upgrade it anyways. The only other choice in list is 'Do Not Upgrade'. I have > 30 A6M2 planes in the pool.

Well I mean is it more of a disadvantage for Japan or for the Allies? With PDU off I'd imagine not every single bomber squadron will be converted to a B-29, and not every single fighter squadron will be P47/P59H/Corsair/P80

I think a lot of people are failing to realise that. Might be due to the fact that most PBEMs don´t get into the later years when the Allied planes actually outclass the Japanese.

I think playing with PDU off could be pretty interesting. Certainly a lot more diversity then just seeing 2-3 types of AC all the time.

OP my newbie conclusion from that particular squadron was that 'units with withdrawal dates cannot be upgraded' - I'm not sure if that is true or not but that was what I concluded. I noticed the same thing on the same squadron.